Three typical external carbon sources (i.e. leachate, hydrolysates from primary sludge and starch solution) with regard to the denitrification process were investigated respectively in sequencing batch reactors (SBR) and their denitrification properties were compared to provide the optimum substrate suited for nitrogen removal processes. The variations of nitrogen were examined and nitrate utility ratio as well as carbon consumption rate was exhibited, denitrification kinetics regarding leachate was also analyzed by use of zero-order kinetic model. The experimental results indicated that the similar denitrification trend was achieved between hydrolysates from primary sludge and starch solution other than leachate as carbon source in denitrification system. The nitrate was not entirely degraded and the nitrite generated permanent accumulation. Compared with other carbon sources, leachate in steady operation showed the highest nitrate removal concentration and rate in phase I with an average of 13 mg/L and 86%, respectively. In phase II, the maximum nitrate and carbon decomposition rates were 0.088 g N/g VSS•d and 0.848 g TOC/g VSS•d respectively occurring at leachate as substrate. Thus, stable leachate was considered as the most suitable carbon source in comparison with other substrates.